This relates to a method and apparatus for data transmission over long distance circuits such as those using earth satellites.
Satellite circuits provide good amplitude and delay responses, low harmonic distortion and a moderate and relatively fixed background noise level. These circuits are also immune from the fading that is typical on microwave circuits.
However, the enormous length of a satellite circuit introduces significant round trip propagation delay (RTPD) into two-way connection circuits. Such delays are typically in excess of 600 msec compared to delays of less than 50 msec encountered on land microwave circuits and less than 100 msec on cable connections. These extremely long delays significantly decrease two-way data throughput since there must be a delay equal in length to the RTPD between the time one terminal ends transmission and the time it begins to receive a response. Moreover, delays affect the timing that must be allowed for acknowledgements and the like that are used in signaling between the terminals interconnected by the circuit. Further, such circuits inevitably have echoes. These echoes are worse for half duplex data sets such as the 202S, 201C and 208B modems widely used in data communications.
Echoes are currently controlled by several methods. In one technique, approximately a 3dB loss is introduced in each transmission path so as to achieve at least a 6dB signal to echo ratio. The magnitude of this loss, however, depends on the total length of the circuit and the loss, of course, also reduces the received signal. Accordingly, this technique is not a good solution for a long haul network.
A second technique is the use of an echo suppressor. As explained at page 82 of Transmission Systems For Communications (Bell Telephone Laboratories, Revised 4th Edition 1971), an echo suppressor is basically a pair of voice operated switches which, while one subscriber is talking insert a loss of 35dB or more in the echo return path. Thus, the suppressor has the effect of changing a two-way four wire connection into a one-way two wire connection. Typically, echo suppressors are installed in a split mode so that half of the job is done at one end of the circuit and the other half at the other end. Such suppressors, however, are designed for voice transmission circuits in a way which introduces significant delay when the circuit is used for data transmission. In particular, it can take from 100 to 150 msec to switch the suppressors from a configuration supporting transmission in one direction to a configuration supporting transmission to the other direction.
Still another device for controlling echoes is the echo canceller which is now being introduced into satellite communication circuits. Echo cancelling is a linear process performed at both ends of the circuit which seeks to null out the echo by subtraction. Because the echo canceller has no prior knowledge of the characteristics of the circuit, the echo canceller must be adaptive; and to adapt the echo canceller to the circuit, a training preamble is required. The use of such a preamble, however, causes a delay of up to 500 msec before the circuit is properly conditioned for communication. Further details concerning echo cancelling techniques are set forth in "Improving Transmission in Domestic Satellite Circuits", Bell Laboratories Record (September, 1977) and D. L. Duttweiler and Y. S. Chen, "A Single Chip VLSI Echo Canceller", Bell System Technical Journal, Vol. 59, No. 2, pp. 149-160 (February 1980).
Alternatively, echoes may be controlled by modifications of the signalling protocols and/or the modem itself. For example, by attaching a unique identification code to the data sent from each modem, echoes of such data can be recognized and suppressed when they return to the modem. In another technique, the modem receiver is de-activated (or squelched) while the modem transmitter is sending and for as long thereafter as there is signal energy on the line. By ensuring that the duration of transmission is longer than the time it takes for the echo to return to the modem, the modem receiver will remain squelched until the echo also ends. If, however, the transmission duration must be artificially extended, data throughput will be reduced. In still another technique, the energy detection threshold of the modem receiver is set at a level which discriminates against the signal energy in the echo.
To cope with the satellite circuit problem, one manufacturer has modified the handshake sequence that is performed at the time that data communication is established between a pair of modems. In the past, this sequence included the following steps:
causing a ringing signal to be applied to the answer modem in response to the initiation of communication from an originate modem;
connecting the answer modem to the communication line after receipt of the ringing signal;
transmitting an answerback tone from the answer modem to the originate modem for two to three seconds after 1.5 to two seconds of silence following connection of the answer modem to the line; and
transmitting data from the answer modem to the originate modem after 50 msec of silence following the answer back tone.
With 201 and 202 type modems, this handshake sequence has been modified to extend the 50 msec period of silence to 500 msec. This period of time is sufficient to reactivate the echo suppressors on the line which would be disabled by the answerback tone. To provide the echo suppressor sufficient time to cut in, a carrier signal is then transmitted for a period of approximately 150 msec and this in turn is followed by the data signal. As will be apparent, the function of the carrier signal is solely to provide a signal to trigger the echo suppressors and other signals could be used for this purpose.
This "sacrificial carrier" technique, as it is called, has several disadvantages. It ignores the possibility that there is an echo canceller instead of an echo suppressor in the line. As a result, it must provide sufficient time for echo suppressor cut in every time the direction of data communication is changed. Where there is a significant amount of data exchange between two terminals, this causes significant loss of throughput. In addition, this technique provides no means for training an echo canceller. As a result, the users of such a communication system must expect polling errors until the echo canceller has trained on the data being sent.
With the 208 modem, a different technique is used, namely, that of ensuring that the duration of transmission in one direction is longer than the round trip propagation delay. This, however, has the disadvantage of requiring the communication system to assume that the round trip propagation delay is 600 msec or more since the system does not know whether the originate and answer modems will be interconnected by satellite or by the shorter land circuits. As a result, the minimum duration of data communication in such a system must be on the order of 600 msec. To guarantee this 600 msec transmission time, the clear to send (CTS) response of the modem must be delayed by the same duration. As will be apparent such a 600 msec request to sent to clear to send delay significantly decrease data throughput.